The invention relates to a milling method for producing any desired finished part from any desired blank by a continuous approach from the shape of the blank to the shape of the finished part by continuous material removal, and to a method of producing guide paths for a milling tool used in the milling method. The methods are used in particular in the production of turbine blades.
The prior art of milling methods is described here with reference to the production of turbine blades.
Turbine blades of various size are produced, for example, from parallelepiped-shaped blanks. For this purpose, first of all pieces having planar surfaces and corners of the blank are milled off in order to achieve an initial rough polygonal approximation to the blade shape. The milling tool is then guided along a plurality of guide paths or guide-path sections which in each case run along equidistant surfaces. By the milling of a plurality of such equidistant surfaces, the shape of the finished part is achieved in steps. In this case, the guide path is defined by discrete points and/or curve sections.
According to this method a blade is produced slice by slice and with frequent lifting and renewed application of the milling tool. The result of this is that the milling tool moves through the air during a significant period of time and no cutting takes place during this time.
A reduction in the machining time by increasing the milling speed is limited due to the associated decrease in the machining time of the milling tools used.
The use of ceramic tools is not possible in this traditional milling method, since the milling paths often lead there to an unsteady and intermittent progress of the machining process due to frequent lifting and application of the tool. Since it is known that ceramics are very sensitive to fracture and the milling tool often experiences blows along the milling path, it is not suitable for this milling method.
The object of the invention is to provide a milling method for producing a finished part of any desired contour from any desired blank, which milling method avoids the disadvantages of the method described at the beginning. It is in particular the aim of the invention to reduce the machining time of the workpiece and to use the milling tools in such a way that they are subjected to less wear.
This object is achieved according to the invention by a milling method in which the milling tool is guided along a spiral guide path which runs from a blank of any desired contour continuously down to the desired finished part of any desired contour. Such a spiral guide path can be placed in a workpiece of any shape, irrespective of the contour of the blank and the finished part.
The milling tool is guided along a guide path of this type, as a result of which a milling path is produced which is likewise continuous and basically spiral. The resulting milling path may differ from the guide path covered by inclining the milling tool, whereby however the continuity is retained.
The spiral guide path permits a continuous change in form with continuous material removal from the workpiece, in which case the tool only has to be applied once. The complete milling operation may therefore be effected in a single set-up of the workpiece.
The spiral guide path is distinguished by a smooth and gentle progression which is free of sudden changes of direction. This is the precondition for harmonic guidance of workpiece and tool, this harmonic guidance being free of sudden accelerations and vibrations associated therewith. To this end, the speed of the tool and the workpiece is adapted to the curvature of the guide path.
The main advantage of the method according to the invention lies in the reduction in the machining time of the workpiece and thus in a more cost-effective production. This may be attributed primarily to the continuous material removal. Compared with known methods, the volume of removal per unit of time is greater, since the tool is not directed through the air very often or is not directed through the air at all. The total machining time is further reduced by the workpiece only being clamped in place once. As a result, only one set of clamping positions is required, which further simplifies the method.
A further advantage lies in the smooth tool guidance, as a result of which shocks on the tool are avoided and consequently the use of ceramic tools is made possible. Thanks to their heat resistance, higher milling speeds can be realized, which leads to a further reduction in the milling time.
Furthermore, the continuous material removal and change in form produce the advantage of less distortion of the workpiece.
The milling method according to the invention can be carried out on existing four- or five-axis machine tools. In this case, the number of axes used depends on the complexity of the surface curvature. Whereas simple parts can be milled like clamping positions on four axes, the production of a twisted turbine blade requires milling along five axes.